1. FIELD OF THE INVENTION
This invention relates to a semiconductor device. More particularly, though not exclusively, it relates to such devices which are heterostructures of narrow bandgap semiconductor materials.
2. DISCUSSION OF PRIOR ART
Narrow bandgap semiconductors such as InSb have useful properties such as very low electron effective mass, very high electron mobility and high saturation velocity. These are potentially of great interest for high speed device applications. Unfortunately it has proved difficult to overcome the drawbacks of these materials. A prior art three terminal active device which is a narrow bandgap semiconductor material heterostructure is described by T Ohashi et al in J Vac Sci Technol B4 622 (1986). It is a thin film depletion mode field effect transistor (FET) device, and consists of an InSb film on a GaAs substrate. However, the device of Ohashi et al unfortunately has poor performance, a high leakage current in particular. It has a dynamic range of only 7dB, and so its current in an ON state is only about twice (5.sup.1/2) that in an OFF state. This exemplifies the difficulty of exploiting the useful properties of narrow bandgap semiconductors.
European Patent Application No.8530405.1-2203 published as No. 0 167 305 corresponding to U.S. Pat. No. 5,016,073 discloses photodiodes having two or more terminals and formed as heterostructures of semiconductor materials. There is no disclosure of field effect transistors or bipolar transistors.